High-strength colored glass ceramics as a cooktop, smooth on both sides

ABSTRACT

A glass-ceramic cooktop is provided that is made of glass-ceramic material with a flat upper side and an underside. The glass-ceramic material has transmittance values of greater than 0.1% in the visible light range in the total wavelength region greater than 420 nm, a light transmittance in the visible range of 0.8-2.5%, and a transmittance of 0-85% in the infrared at 1600 nm, and wherein the glass-ceramic material has high quartz mixed crystals as the prevalent crystal phase. The underside is flat, unstructured, and coplanar with the upper side.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims benefit under 35 U.S.C. §119(a) of German PatentApplication No. 10 2011 050 867.8, filed Jun. 6, 2011, the entirecontents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a glass-ceramic cooktop having a flat upperside forming a cooktop and a smooth underside, wherein the glass-ceramicmaterial of the cooktop has transmittance values of greater than 0.1% inthe visible light range in the total wavelength region greater than 420nm, a light transmittance in the visible range of 0.8-2.5%, and atransmittance of 0-85% in the infrared at 1600 nm, and wherein theglass-ceramic material has high quartz mixed crystals as the prevalentcrystal phase.

2. Description of Related Art

Cooktops having a glass-ceramic plate as a cooking surface are familiarprior art. These glass-ceramic plates are usually present as flat platesor are shaped three-dimensionally.

Glass ceramics with high quartz mixed crystals as the prevalent crystalphase are produced from crystallizable lithium aluminum silicateglasses.

These glass ceramics are produced in several steps.

In the large-scale technical production of glass ceramics, first thecrystallizable initial glass made up of a mixture of shards andpowder-form batch raw materials is melted at temperatures usuallybetween 1500 and 1650° C. Typically, arsenic and/or antimony oxide isused as a refining agent in the melt. These refining agents arecompatible with the required glass-ceramic properties and lead to goodbubble qualities of the melt. Although these substances are rigidlybound in the glass framework, they are a disadvantage from the points ofview of safety and environmental protection. Thus, special precautionarymeasures must be taken in the recovery and treatment of raw materialsand due to evaporation in the melt.

Recently, the particular use of SnO₂ as an unobjectionable refiningagent has been described. In order to obtain good bubble qualities, atconventional melting temperatures (a maximum of approximately 1680° C.),in addition to SnO₂ preferably halide compounds are used as additionalrefining agents. Thus, the use of 0.1-2 wt. % SnO₂ and 0-1 wt. % Cl isdescribed in the Japanese Patent Applications JP 11 100 229 A and JP 11100 230 A. According to these publications, coloring by addition of V₂O₅as the only colorant has been achieved.

The addition of 0.05-1 wt. % fluorine (US 2007 0004578 A1) and 0.01-1wt. % bromine (US 2008 0026927 A1) for support of refining with SnO₂ isalso disclosed. Refining temperatures below 1700° C. have also beendescribed in these publications. The primary colorant is V₂O₅. Theaddition of halides is a disadvantage, since they vaporize greatly atthe melting temperature and thus form toxic compounds, such as, e.g.,HF.

The use of SnO₂ in combination with high-temperature refining above1700° C. in order to obtain good bubble qualities is described in DE 19939 787 C2. This publication, however, provides no indication forobtaining a good display capability in the wavelength range startingfrom 420 nm.

After melting and refining, the glass usually undergoes a hot shaping byrollers or more recently floats, in order to produce plates. For aneconomical production, on the one hand, a low melting temperature and alow processing temperature V_(A) are desired; on the other hand, theglass should not show any devitrification during the shaping. That is,no disruptive crystals that would adversely affect the strength in theinitial glasses and the glass ceramics produced therefrom should beformed. Since shaping takes place in the vicinity of the processingtemperature V_(A) (viscosity of 10⁴ dPas) of the glass, it must beassured that the upper devitrification temperature of the melt lies inthe vicinity of and most favorably below the processing temperature, inorder to avoid the formation of disruptive crystals.

Subsequently, the initial glass is converted into the glass-ceramicarticle by controlled crystallization. This ceramicizing takes place ina two-step temperature process, in which first nuclei are produced bynucleation at a temperature between 680 and 800° C., usually fromZrO₂/TiO₂ mixed crystals. SnO₂ can also participate in the nucleation.With subsequent increase in temperature, the high quartz mixed crystalsgrow on these nuclei. High rates of crystal growth, such as are desiredfor an economical, rapid ceramicizing, are obtained at temperatures of850 to 950° C. For this maximum production temperature, the structure ofthe glass ceramics is homogenized, and the optical, physical andchemical properties of the glass ceramics are adjusted. If desired, thehigh quartz mixed crystals can subsequently still be converted intokeatite mixed crystals. The transformation into keatite mixed crystalsis produced in the case of an increase in temperature in a range fromapproximately 950 to 1200° C. With the transition from high quartz tokeatite mixed crystals, the thermal expansion coefficient of the glassceramics is increased and the transparency is reduced due to the lightscatter that accompanies the enlargement of the crystals. As a rule,glass ceramics with keatite mixed crystals as the principal phase arethus translucent or opaque and the light scatter associated therewithacts negatively on the display capability.

A key property of these glass ceramics with high quartz mixed crystalsas the principal crystal phase is the ability to produce materials thatprovide an extremely low thermal expansion coefficient of <0.5×10⁻⁶/K inthe range from room temperature up to 700° C. and above. Based on thelow thermal expansion, these glass ceramics possess an excellentresistance to differences in temperature and stability relative tofluctuating temperatures.

In the application as a cooktop, the technical development based onrequirements from practical use leads to very specific, partiallycontradictory requirements for transmittance.

In order to prevent a disruptive view onto the technical componentsbelow the glass-ceramic cooktop and in order to avoid the dazzlingeffect due to radiating heating elements, in particular bright halogenheating elements, glass-ceramic cooktops are limited in their lighttransmittance. For display capability, however, a certain lighttransmittance is necessary in order to assure sufficient brightness withthe use of commercial components, e.g., signal generators, LEDs, etc. Inorder to satisfy these requirements, glass-ceramic cooktops are usuallyadjusted to light transmittance values of 0.5 to 2.5%. This is achievedby additions of coloring elements. Glass-ceramic cooktops then appear tobe black in a top view, due to the low light transmittance, no matterwhat the coloring element used, while in a transparent view, they appearfor the most part red, red-violet or orange-brown according to thecoloring elements used.

Color displays are composed of electronic components emitting light, forthe most part light-emitting diodes, which are incorporated below thecooktop. They are particularly necessary in the case of inductioncooktops for ease of operation and safe operation. For example, theactual heating power or residual heat of the different cooking zones isoptically displayed. The display of the residual heat is important forsafe handling when the heating elements are not turned on or, as in thecase of inductively heated cooking surfaces in general, it cannot beascertained that the cooktop is hot. The usual red light-emitting diodesirradiate at wavelengths of around 630 nm. In order to improve ease ofoperation and technical functions, but also to offer the possibility forhousehold appliance manufacturers to differentiate their designs, inaddition to the usual red display, displays of other colors are alsodesired.

Cooktops of Japanese origin are known, in which an LCD display ispresent, which can be backlit in green, orange and red.

The most varied colors that are used herewith, with the exception of thecolor red, at the present time serve exclusively for esthetic purposes.The color red, however, usually always indicates danger.

Safety information is only coded and known via display elements orsymbols of the same color in seven-segment displays. In safety-criticalsituations, the user is forced to think of which display he wants toturn on. Added to this is the fact that there is a flood of informationavailable to the user due to the high degree of technology in kitchensand the many appliances present in kitchens, such as cooking ovens,baking ovens, microwaves, grilling devices, hoods, refrigerating andfreezing appliances, as well as bread slicing machines, etc., theinformation being different from one device to another. For example, ared blinking light in one appliance can indicate a danger, while inanother device, it indicates an operation.

In commercial colored cooktops, the user cannot recognize the operatingstate and error condition by means of colors, thus whether the applianceis ready to operate or whether an indication of a possible errorcondition is present.

An earlier type of glass-ceramic cooktop, known under the name CeranColor®, produced by SCHOTT AG, possessed good color display capability.Ceran Color® is colored by additions of NiO, CoO, Fe₂O₃ and MnO andrefined by Sb₂O₃. A light transmittance of typically 1.2% is adjustedfor cooktops with a usual thickness of 4 mm by this combination of coloroxides. The transmittance in the range of 380 nm to 500 nm is 0.1-2.8%,depending on wavelength in each case. In the case of a wavelength of 630nm that is common for red light-emitting diodes, the transmittanceamounts to approximately 6%. It is a disadvantage in this earlier typeof glass-ceramic cooktop that the color oxides used also absorb verystrongly in the infrared. The IR transmittance at 1600 nm amounts toless than 20%. Thus, the rate of cooking is reduced. The transmittancecurve of Ceran Color® is illustrated in the book “Low Thermal ExpansionGlass Ceramics”, Editor Hans Bach, Springer Publishing Co. BerlinHeidelberg 1995, on page 66 (ISBN 3-540-58598-2). The composition islisted in the book “Glass-Ceramic Technology”, Wolfram Hõland and GeorgeBeall, The American Ceramic Society 2002 in Tables 2-7.

In more recent, further developed glass-ceramic cooktops, for the mostpart V₂O₅ is used for coloring, since it has the special property ofabsorbing in the visual light range and permitting a high transmittancein the range of infrared radiation.

The coloring by V₂O₅ is represented as a very complex process. As wasshown in earlier investigations (DE 19939787 C2), a redox process is aprerequisite for converting the vanadium oxide to the coloring state. Incrystallizable initial glass, the V₂O₅ still colors relatively weaklyand produces a light green color shade. In the ceramicizing, the redoxprocess occurs, the vanadium is reduced and the redox partner isoxidized. The refining agent functions as the primary redox partner.This was shown by Mõssbauer investigations of Sb and Sn-refinedcompositions. In the ceramicizing, a part of the Sb³⁺ or Sn²⁺ in theinitial glass is converted to the higher oxidation state Sb⁵⁺ or Sn⁴⁺.It can be assumed that the vanadium is incorporated in the seed crystalin the reduced oxidation state as V⁴⁺ or V³⁺ and is intensively coloredtherein due to electron charge-transfer reactions. Also, as anotherredox partner, TiO₂ can reinforce the coloring by vanadium oxide. Inaddition to the type and quantity of the redox partners in the initialglass, the redox state that is adjusted in the glass for the melt alsohas an influence. A lower oxygen partial pressure pO₂ (melt adjusted asreducing), e.g., due to high melting temperatures, reinforces thecoloring effect of the vanadium oxide.

The ceramicizing conditions have another influence on the coloringeffect of the vanadium oxide. In particular, high ceramicizingtemperatures and longer ceramicizing times lead to a more intensecoloring.

The described relationships for coloring by means of V₂O₅ will be usefulfor the person skilled in the art, in order to establish the desiredtransmittance curve by means of a specific glass composition, specificredox adjustments of the pO₂ for the melt and the ceramicizingconditions. Previously, however, it was not possible to achieve allrequirements, such as light transmittance and high IR transmittance incompliance with specifications, as well as display capability forstandard red light-emitting diodes together with the desired improveddisplay capability for light-emitting displays of other colors.

The form of the absorption bands of the vanadium oxide and thustransmittance in the visible light range in the entire wavelength regiongreater than 450 nm up to the upper limit of 750 nm could not be adaptedto higher transmittances.

Examples of such types of V₂O₅-colored glass-ceramic cooktops are theSb₂O₃-refined Ceran Hightrans® and the SnO₂-refined Ceran Suprema®,which are produced by the company SCHOTT AG. The transmittance curves ofthese two glass ceramics are published in the book “Low ThermalExpansion Glass Ceramics”, Second Edition, Editor Hans Bach, DieterKrause, Springer Publishing Co. Berlin Heidelberg 2005, on page 63 (ISBN3-540-24111-6).

The transmittance value of 0.1% is not exceeded in the case of the namedglass-ceramic cooktops and for other glass-ceramic cooktops found on themarket in the wavelengths of approximately 450-550 nm that are importantfor the visibility of color displays, in particular blue and greendisplays. Other essential requirements for transmittance are fulfilledby these glass-ceramic cooktops: high infrared transmittance for highrates of cooking, transmittance in compliance with specifications forstandard red light-emitting diodes at approximately 630 nm and a lighttransmittance of about 1.5%.

In order to eliminate this disadvantage, the European Patent ApplicationEP 1465460 A2 discloses a glass-ceramic cooktop that has a Y value(brightness) of 2.5-15 for a thickness of 3 mm, measured in the CIEcolor system with standard light C. The designations “brightness” andlight transmittance correspond to the same measurement value. The Yvalue is identical to the value of light transmittance, measuredaccording to DIN 5033. Improved displays for blue and greenlight-emitting diodes will be obtained with this light transmittance.The disclosed compositions are refined with As₂O₃ and/or Sb₂O₃,partially in combination with SnO₂. The coloring is carried out by meansof V₂O₅.

It is pointed out in the comparative example that the display capabilityfor blue and green light-emitting diodes having the listed materialcompositions is insufficient for a light transmittance of 1.9%. Theclaimed high values of light transmittance of at least 2.5% andpreferably higher, however, are disadvantageous with respect to hidingthe electronic components underneath the cooktop. In addition, theesthetic black appearance of the cooktop from a top view is adverselyaffected.

A cooktop of glass-ceramic material is known from DE 10 2009 013 127 A1,which provides transmittance values of greater than 0.1% in the visiblelight range in the total wavelength region greater than 420 nm, a lighttransmittance in the visible range of 0.8-5%, (preferably 0.8-2.5%) anda transmittance of 45-85% in the infrared at 1600 nm. With such acooktop, it is assured that the disruptive transparent view onto thetechnical components underneath the glass-ceramic cooktop is preventedand the esthetic black appearance in the view from the top remainsassured. Radiant heating elements are visible during operation andcommon red light-emitting diode displays can be well recognized. Due tothe transmittance of more than 0.1% in the visible light range in thetotal wavelength region greater than 450 nm, displays of other colorsare also well recognizable. In view of the luminosity of commercialblue, green, yellow or orange light-emitting diodes, this transmittancevalue is sufficient and represents a clear improvement when comparedwith the prior art. In particular, displays having blue and green colorsare clearly improved. Displays with white light are less falsified incolor due to the transmittance curve in the entire wavelength regionabove 450 nm.

The glass-ceramic plates for the cooktops are shaped by upper and lowerrollers via a special rolling process. The melted liquid initial glassis introduced into the rollers via a drawing nozzle. The rollers arecomposed of a special material in order to assure a controlled heatextraction between glass and rollers. Uncontrolled crystallization inthe molds of the glass strip must be avoided during the hot shaping bythe rollers. The glass strip is guided over a roller table into anannealing lehr. The glass strip is initially kept at temperatures higherthan the transformation temperature and lower than the nucleation andcrystallization temperatures of the initial glass, in order to reducepossible stresses. After cooling the glass strip to room temperature,the glass strip is cut, the edges are processed, it is decorated withceramic colors and subsequently transformed into glass ceramics in theceramicizing oven.

These glass-ceramic plates for cooktops possess a knob-like undersidestructure in order to fulfill the strength requirements forglass-ceramic plates for cooktops. These knobs are embossed in theunderside of the glass strip via a knobby bottom roller during the hotshaping.

This knobby structure is composed of regular patterns of spherical capsor knobs that are round or oval or can also be of another shape. Theknobs bring about a protection of the underside of the glass-ceramicplate against strength-reducing lesions.

The strength is lastly obtained in that the “lesions” of the undersideare collected on the knob caps, so that in “valleys”, where the maximumdangerous tensile stresses occur under load, the notching effect isreduced, since the glass-ceramic surface has not been damaged.

The disadvantage of these knobs is the scattering of the light that isconducted through the glass-ceramic plate. It is not possible to makevisible without distortion the displays or structures underneath theglass-ceramic plate. Displays and also the cooking zones are thusperceived in a slightly distorted manner.

The local introduction of a silicone layer in order to make visiblewithout distortion the usual light-emitting displays is known from DE 4104 983 C1. This silicone layer, however, introduces an additionalexpense, has poor transmittance behavior, and is less temperature-stableat the high heating temperatures of the cooktop. For this reason, thisimmersion layer can just be used locally in cold regions of the cooktop.The distorted view of the heating zones still exists.

BRIEF SUMMARY OF THE INVENTION

The object of the invention is to create a glass-ceramic cooktop of thetype described initially, which is characterized by improved applicationproperties, for example, good display qualities of the display devices,and/or the functionality of heating elements and/or sensor units.

This object is achieved by the fact that the underside is shaped flat,unstructured and coplanar with the upper side.

Accordingly, it is proposed according to the invention that theunderside of the cooktop, like the upper side of the cooktop, is notstructured and is shaped flat. Therefore, it does not have the familiarknobs known from the prior art, but is as smooth as the surface. Gooddisplay properties can be provided by this coplanar arrangement of theupper side and the underside. In particular, in combination with theglass-ceramic material used, distortion-free color (e.g., blue)displays, which were not previously possible, having a clearly improveddisplay sharpness can be provided.

According to a preferred variant of the invention, it can be providedthat at least one coating and/or film is introduced onto the underside.Due to the fact that the underside is shaped flat and smooth, coatingswith a uniform thickness can also be provided, which then alsosimultaneously have uniform properties. The coating and/or film in thiscase can be part of a display. In particular, the coating can form amasking having light-transparent and non-transparent regions. Thismasking is then disposed between the cooktop and a light-emittingelement, whereby the light-emitting element transmits its light throughthe transparent regions into the cooktop and thus the light can then bedecoupled on the upper side of the cooktop, and in fact, withoutdistortion, a sharp contour display corresponding to the masking beingprovided.

The coating and/or film forms a masking with light-transparent andnon-transparent regions and a light-emitting element, e.g., a 7-segmentdisplay or a display unit, is disposed at a distance in the region underthe coated underside.

It has been shown that with the smooth configuration of the cooktop onboth sides according to the invention, in combination with theglass-ceramic material used, cooktop thicknesses in the range between 2mm and 6 mm, preferably in the range between 3 and 5 mm, can beprovided. In this way, a sufficient mechanical stability results forcooktop applications.

It has been shown that the thickness of the coating should lie in therange between 100 nm and 2 mm.

The coating is particularly preferred to be temperature-stable at leastup to 85° C. In this case, the coating is particularly suitable for theregion of the displays and operating elements. It can even itself forman electrically conductive heating element.

A simple manufacture is then possible, if the coating is a sol-gelcoating or an ITO coating. For example, with ITO coatings astemperature-dependent materials, locally resolving contact sensors instructured form or unstructured temperature sensors over a large surfacearea can be provided. It is also conceivable that a silicone coating isused as the coating.

A conceivable variant of the invention is that the back side of thecoating that is not facing the underside of the cooktop is provided witha structuring, in particular a mechanical surface deformation or athermally embossed deformation. The coating properties can be expandedby means of this modification of this coating. For example, the coatingcan be structured by mechanical deformation, for example with the use ofembossing rollers. It is also conceivable that the coating is influencedthermally in a targeted manner, for example by use of lasers, or thatthe coating is etched. The coating can be formed, for example, as anelectrically insulating layer or it may also be electrically conductive.With electrically conductive coatings, induction coils can be formed,for example, which then form the heating elements. The smooth undersideof the cooktop makes possible a uniform coating thickness, so that theconductor path of the induction coil forms a uniform current-conductingcross section. Adjacent to the induction coil that has been introduced,an electrically insulating layer can be applied onto the underside ofthe cooktop.

Another variant of the invention is that the coating is an electrode ofa contact-sensitive sensor (touch sensor), a pot or pan sensor or apot-size sensor. Such electrodes can be shaped in structured form aselectrodes for touch sensors, in particular in the field of theoperating region to hide the touch electronics, in particular over theentire surface area of the cooktop in order to make possible, e.g.,touch functionalities also in the direct vicinity of the cooking zones.The electrode design can be executed via at least two separateelectrodes, so that it can distinguish between a finger contact and alarger pan bottom. In addition, it is conceivable that the electrodesare designed structured over their entire surface area as pot or pansensors and pot-size sensors.

In addition, it is conceivable that the coating forms a thermalinsulation. In this way heat transports for the purpose of saving energycan be eliminated in a targeted manner.

A particularly preferred variant of the invention is one such that thesurface roughness of the underside of the cooktop is R_(a)≦5 mm, in theusual scanning region. With this surface roughness, on the one hand,contour-sharp displays can be produced, in particular if they aremounted at a distance. On the other hand, such a surface roughness formsa sufficiently strong binding for the coating.

A particularly preferred embodiment of the invention is characterized inthat the cooktop is surrounded by a frame and that the frame holds theunderside in the edge region of the cooktop. The smooth underside of thecooktop makes possible a particularly good seal between the cooktop andthe frame.

BRIEF DESCRIPTION OF THE DRAWING

The sole FIGURE is a schematic representation, in the lateral view, of acooktop.

DETAILED DESCRIPTION OF THE INVENTION

The invention will be explained in further detail in the following onthe basis of the example of embodiment that is shown in the drawing. Inschematic representation and in the lateral view, this drawing shows acooktop 1, which is composed of a glass-ceramic material. Cooktop 1 hasan upper side 2 and an underside 3. Both upper side 2 as well asunderside 3 are formed smooth approximately with the same surfacestructure. This means that, in particular, underside 3 does not have theusual periodically repeating knobby structures. Therefore, upper side 2and underside 3 form two coplanar surfaces. A coating 4 and/or a film 4is introduced on underside 3. Here, for example, a screen printingmethod, a sputtering or an injection molding method can be used. Coating4 is formed here by a transparent material. A light-emitting element 5,for example an LED, is coupled to cooktop 1 on back side 8. Thislight-emitting element 5 passes its light through coating 4 and throughcooktop 1. The light then exits from upper side 2 of cooktop 1. For theformation of a display device, coating 4 can be marked, for example,with regions that are not light-transparent.

As the FIGURE further shows, coating 4 can also be formed in anelectrically conducting manner as an electrode, this electrode thenbeing part of a contact-free sensor (touch sensor). Coating 4 can becontacted at an electronic control 7 by means of a contact 6, which canbe formed, for example, from an electrically conductive foam. Theelectrically conductive coating 4 serving as an electrode can bedisposed, for example, in the field of the operating region hiding thetouch electronics (electronic control) 7. In particular, it is alsopossible that the coating extends over a large surface area, inparticular over the entire surface area, over underside 3 of cooktop 1,in order to make possible, e.g., touch functionalities also in thedirect vicinity of the cooking zone.

Within the scope of the invention, it is also conceivable to use IRtouch sensors in the region of underside 3 of the cooktop. The IRsensors are disposed in the region of underside 3. Because of the smoothformation of underside 3 of the glass ceramics, a constant noise levelcan be provided. Consequently, this increases the sensitivity anddecreases the interference or noise susceptibility of the IR sensors.

What is claimed is:
 1. A glass-ceramic cooktop comprising: aglass-ceramic material having a flat upper side forming a cooktop and anunderside, the underside bring flat, unstructured, and coplanar with theupper side, wherein the glass-ceramic material has a transmittance valueof greater than 0.1% in the visible light range in the total wavelengthregion greater than 420 nm, a light transmittance in the visible rangeof 0.8-2.5%, and a transmittance of 0-85% in the infrared at 1600 nm,and wherein the glass-ceramic material has high quartz mixed crystals asthe prevalent crystal phase.
 2. The glass-ceramic cooktop according toclaim 1, wherein the glass-ceramic material has a thickness in a rangebetween 2 mm and 6 mm.
 3. The glass-ceramic cooktop according to claim1, wherein the glass-ceramic material has a thickness in a range between3 mm and 5 mm.
 4. The glass-ceramic cooktop according to claim 1,further comprising at least one coating on the underside.
 5. Theglass-ceramic cooktop according to claim 4, wherein the coating is partof a display.
 6. The glass-ceramic cooktop according to claim 5, whereinthe coating forms a masking with light-transparent and non-transparentregions.
 7. The glass-ceramic cooktop according to claim 6, furthercomprising a light-emitting element disposed at a distance from theunderside in the region of the coating.
 8. The glass-ceramic cooktopaccording to claim 4, wherein the coating has a thickness in a rangebetween 100 nm and 2 mm.
 9. The glass-ceramic cooktop according to claim4, wherein the coating is temperature-stable at least up to 85° C. 10.The glass-ceramic cooktop according to claim 4, wherein the coating is amaterial selected from the group consisting of metallic, metal-oxidic,inorganic, organic, and nitritic.
 11. The glass-ceramic cooktopaccording to claim 4, wherein the coating has a backside that not facingthe underside, the backside comprising a mechanical surface deformationor a thermally embossed deformation.
 12. The glass-ceramic cooktopaccording to claim 4, wherein the coating is electrically insulating.13. The glass-ceramic cooktop according to claim 4, wherein the coatingis electrically conductive.
 14. The glass-ceramic cooktop according toclaim 4, wherein the coating forms an induction coil.
 15. Theglass-ceramic cooktop according to claim 4, wherein the coating is anelectrode of a sensor.
 16. The glass-ceramic cooktop according to claim15, wherein sensor is a sensor selected from the group consisting of acontact-sensitive sensor, a pot or pan sensor, and a pot-size sensor.17. The glass-ceramic cooktop according to claim 4, wherein the coatingcan be inductively activated.
 18. The glass-ceramic cooktop according toclaim 4, wherein the coating is thermally insulating.
 19. Theglass-ceramic cooktop according to claim 1, wherein the underside has asurface roughness of less than or equal to 5 mm.
 20. The glass-ceramiccooktop according to claim 1, further comprising a frame that enclosesthe underside in an edge region of the cooktop.
 21. The glass-ceramiccooktop according to claim 1, wherein the glass-ceramic material has anexpansion coefficient of less than or equal to 2·10⁻⁶ 1/K° in atemperature range of 20-700° C.